The present invention relates to a radio handset for detecting its current position using a radio signal and in particular, to a radio handset and a position location system for accurately identifying its current position even when it is difficult to discriminate a base station from which the signal has been transmitted.
In a mobile communication system, there has been suggested a technique to detect a position of a radio handset by using a signal transmitted from a base station. For example, JP-A-7-181242 (laid open on Jul. 21, 1995) discloses a technique for detecting a handset position by using positions of base stations and propagation delay time differences between signals transmitted to a handset from different base stations in a code division multiple access (CDMA) system.
For example, in a radio communication system shown in FIG. 11, a radio handset 800 receives signals from three cellular phone base stations 801, 802, and 803. The radio handset 800 calculates signal propagation time differences, i.e., [propagation time of the signal transmitted from the base station 801—propagation time of the signal transmitted from the base station 802] and [propagation time of the signal transmitted from the base station 801—propagation time of the signal transmitted from the base station 803]. Using the propagation time differences, the radio handset 800 calculates a current position of the radio handset 800.
Here, explanation will be given on a system based on the EIA/TIA-95 specified by the TIA (Telecommunication Industry Association) which is an US CDMA digital moving communication system. In the EIA/TIA-95 system, all the base stations use the same spread code (PN code). Moreover, each of the base stations has a pilot PN offset (PN offset value) as a transmission time difference predetermined for each of the base stations for identification thereof and transmits a signal at a timing shifted from the standard timing by its PN offset value.
A radio handset calculates correlation between the received signal and the PN code so as to obtain a delay profile. For example, a correlation value between the received signal and the PN code is calculated by using a matched filter, the matched filter outputs a delay profile as shown in FIG. 12.
In FIG. 12, 901. 902, and 903 shows correlation values corresponding to signals received from the base stations 801, 802, and 803, respectively. Here, since the base stations have different PN offset values, the correlation values corresponding to the signals from the base stations are temporally separated from one another. Accordingly, in the delay file, it is possible to isolate each of the signals transmitted from the different base stations as well as received timings of the signals from the base stations. Thus, it is possible to detect a location by using the signal propagation time from the base stations.